1. Field of the Invention
The present invention relates to a material for making marine structure and more particularly to a marinestructural material preferably used as a material for making a floating structure such as a fish-cultivating frame, a floating pontoon, a water surface-partitioning fence, a breakwater net or the like which are required to be water-proof, wave-resistant, tide-resistant, corrosion-resistant to the salt contained in sea-water, weather resistant, light, and strong.
2. Description of the Related Arts
In order for a marine-structural material to be durable for a long time, it is particularly important that the marine-structural material is not horizontally deformed against tides and vertically deformed against waves. That is, the marine-structural material must be strong enough to hold its original configuration in a horizontal direction (rigidity in a horizontal direction) against tides and have a high breaking extension so as to follow vertically against waves.
In addition, the marine-structural material is required to be corrosion-resistant to salt contained in sea-water, weather resistant, and wear-resistant light buoyant, strong, and inexpensive and processable.
A marine-structural material having the abovedescribed qualities is strongly demanded when used in a fish-cultivating frame or a pontoon.
Fish cultivating frames include a floating material enclosing a certain area of sea water and a net, having a bottom and hanging from the floating material so as to cultivate fish in the volume surrounded by the floating material and the net. Bamboo bars, wood bars or iron bars provided with floats are used in the fish-cultivating frame.
A fish-cultivating frame having bamboo or wood is normally small, namely, 15 m square. The frame is rigid but sufficiently flexible to follow the tide and waves. These frames are varied in size of limited and strength. Further, the bamboo or wood frame is corroded by sea water in a short period of time and thus it is difficult to use a large fishcultivating frame for a long time. Thus, the bamboo or wood frame can be used as a coastal cultivating fish-cultivating frame in a bay in which waves are quiet, but cannot be used in an off-shore region.
Large fish-cultivating frames having iron bars provided with floats such as a 20 m square frame, a 40 m-diameter circular frame, and a polygonal frame have been proposed. Since the fish-cultivating frame using iron bars is corroded in a short period of time and is heavy, it is necessary to provide it with large floats. Further, the iron bars are rigid with a limited elastic deformable range. A large fish-cultivating frame having iron bars is incapable of following the swelling motion of waves, and is not durable because stress is repeatedly applied to the iron bar. A larger bar is required to prevent the frame from being deformed. As a result, it is necessary to provide the fishcultivating frame with larger floats. Thus, the fishcultivating frame using iron bars can be used as a coastal cultivating fish-cultivating frame in a bay in which waves are quiet, but cannot be used in an off-shore region.
Fish-cultivating frames for use in an off-shore region have been proposed in Japanese Patent Laid-Open Publication Nos. 60-34121 and 62-171627. These fish-cultivating frames follow the swelling motion of waves, which is strongly demanded. According to one proposed frame, bars each having a plurality of elastic circular hoses are connected to each other by joints. According to the latter proposed frame, bars each having a plurality of rigid materials are connected to each other by joints using flexible members.
These fish-cultivating frames of the above construction are capable of following the swelling motion of waves, but need reinforcing members, such as elastic circular hoses, which are utilized as the bars thereof. Therefore, the fish-cultivating frame is not rigid and has a low flexural rigidity in a horizontal direction. As a result, the ability these fish-cultivating frames to hold their original configuration is inferior, and thus the frames tend to be deformed.
Normally, Young's modulus E (flexural elastic modulus) of an elastic member reinforced with synthetic fiber is 2 to 3 kg/mm.sup.2 and approximately only 5 kg/mm.sup.2 even though elastic modulus thereof is great. The elastic modulus of iron is 20000 kg/mm.sup.2.
The bar of the fish-cultivating frame including the reinforcing elastic member has a small in its elastic modulus and is thus deformable in a horizontal direction. In order to increase the frame's ability to hold its original configuration by reducing the degree of deformation, it is necessary to increase the sectional area of each bar so as to increase the shape factor (geometrical moment of inertia) thereof, i.e., increase the value of the flexural rigidity E.times.I thereof.
However, the geometrical moment of inertia I depends on the sectional configuration of the bar. That is, in a solid circular bar which is (d) in diameter, I=.pi.d.sup.4 /64. In a hollow pipe where d.sub.1 is the inner diameter and d.sub.2 is the outer diameter z=.pi.(d.sub.2.sup.4 -d.sub.1.sup.4)/64. Accordingly, in order to increase the geometrical moment of inertia, the sectional area of the bar becomes large, which leads to a large fish-cultivating frame that is expensive.
A pontoon is utilized as an anchorage of a yacht or a motor boat, as an unloading facility in a region where the difference between ebb and flow is great, and as a landing facility.
Generally, a pontoon includes frames each consisting of steel elements welded to each other in a truss configuration with a float made of concrete. Frame structures composed of wood combined with each other or with concrete have been proposed.
However, it is difficult for the pontoon of the above structure to follow the swelling motion of waves in a harbor in which waves swell greatly. In addition, the pontoon is not corrosion-resistant to sea-water.
Generally, pontoons used in an unloading facility or landing facility include a box made of iron covered with concrete. Thus, they are very expensive.
As described above, the following two structures are considered to improve durability of fish-cultivating frames pontoons and the like by allowing them to follow the swelling motion of waves.
1) The entire frame is rigid to prevent the bars defining the frame from being deformed vertically against waves, so that the frame is durable.
According to this structure, the rigidity of the bars in the horizontal direction is increased to improve the frame's performance for holding itself against tides.
However, steel pipes which have a large diameter are required. As a result, the frame is heavy and expensive.
2) The entire frame or a part thereof is flexible so that the entire frame follows the swelling motion of waves because the frame is deformed vertically.
In this case, the rigidity of the frame becomes low these decreasing the ability of the frame to hold its original configuration in the horizontal direction against tides.
The marine-structural material used to build fish-cultivating frames and pontoons are required to have the following requirements. As detailed below, conventional marine-structural material does not satisfy all of these requirements.
1) The marine-structural material should be hard and have a high flexural rigidity.
That is, it is necessary that the marine-structural material have the ability to hold its original configuration (rigidity in a horizontal direction) on the sea against tides. When the fish-cultivating frame is deformed, the volume for cultivating fish becomes small and as a result, the fish trapped within the net by collapse of the frame are injured or killed due to the lack of oxygen.
2) The marine-structural material must be durable.
The frame of the marine-structural material must have a high breaking extension that favorably follows the swelling motion of waves in a vertical direction. The marine-structural material must be resistant to flexural fatigue. In recent years, the location for fish-cultivation are moving more and more from areas near the coast to areas offshore. Therefore, a fish-cultivating frame is required to be able to favorably follow the swelling motion of waves in a vertical direction. The pontoon is also required to have similar performance characteristics for favorably following the swelling motion of waves in a vertical direction.
3) The marine-structural material must be light and corrosion-resistant to sea-water.
4) The marine-structural material must be easily shaped into a required configuration by molding and must be inexpensive.
The cost of the material must be sufficiently low to allow for economical building of large structures. The side of even a small fish-cultivating frame is 15 m and the frame of the pontoon is similarly large.